1. Field of Invention
The invention concerns a blown film extrusion device with a blow head and an external cooling ring mountable on its top, having an annular support surface on its bottom and centerable relative to the blow head via centering devices.
2. Description of the Prior Art
Such a device is known from DE 33 36 181 A1. The so-called blow head has an annular nozzle gap from which plastic melt emerges. Air is blown into the center of the annular nozzle. At the same time air is fed from the outside by means of an external cooling ring against the film tube emerging at the annular nozzle. The external cooling ring is then mounted with an annular surface on its bottom on an annular surface on the top of the blow head so that the external cooling ring can be slightly raised in order to facilitate access to the annular nozzle during maintenance. A problem here is that the already slight eccentricities or oblique positions of the mounted external cooling ring mean that the bubble taken off from the annular nozzle is slanted, which can lead to nonuniform thickness distribution in the film and to problems during further processing.
The centering devices proposed in the known blown film extrusion device are screws, which are aligned on the bottom of the external cooling ring in the radial direction and mounted in a threaded hole. They can be set with their tip against the outer periphery of a circular section in the upper area of the blow head.
Centering occurs in a blown film extrusion device according to DE 92 14 647 U1 through a shoulder with the shape of a truncated cone on the blow head, on which the outer cooling ring is mounted with a conical hole.
It is also known to form centering devices by hook-like angles. The upper edge of the blow head can be enclosed by at least three angles on the bottom of the outer cooling ring so that the centering ring is centered relative to the blow head.
All known blown film extrusion devices have the common feature that their centering devices do not permit temperature-independent centering. Centering of the cooling ring and blow head free of play performed at room temperature is not effective in operation, since the stronger heating of the blow head leads to relatively greater elongation than in the cooling ring. Significant pressure forces would therefore act on the centering devices that are supported on the outer periphery of parts of the blow head. For this reason, centering must be conducted separately at each operating point in the known blown film extrusion devices.